Every major scale has a relative minor and vica versa. The relative minor of a given major scale is built upon the 6th note of the major scale. Thus in the C major scale, A is the 6th note and A minor is the relative minor.
If you compare the A minor "pure" scale with the C major scale you will notice that they both contain the same notes (i.e. no sharps or flats), thus, the scales are said to be "relative".
In this book, 4 major/relative minor scales are studied and these are listed below:
(Although the key of A major is also studied, its relative minor, F#m, is beyond the scope of this text).
The major key and its relative minor both share the same key Signature.